FIG. 10 shows a conventional ferroelectric capacitor. A silicon oxidation layer 4 is formed on a silicon substrate 2. A lower electrode 6 made of platinum is provided thereon. A PZT (PbZrxTi1−xO3) film 8 as a ferroelectric layer is formed on the lower electrode 6, and an upper electrode 10 made of platinum is provided thereon. Thus, the ferroelectric capacitor is formed by the lower electrode 6, the PZT film 8 and the upper electrode 10.
The reason to use platinum for the lower electrode 6 is as follows. The PZT film 8 must be formed on a layer which can be oriented. The ferroelectricity of PZT is degraded if the PZT film can not be oriented, for example, when the PZT film is formed on an amorphous layer. Meanwhile, the lower electrode 6 must be formed under insulation from the silicon substrate 2. As such, the silicon oxidation layer 4 is formed on the silicon substrate 2. The silicon oxidation layer 4 is made of amorphous material. In general, although a layer formed on an amorphous material becomes nonorientable, a layer made of platinum has a characteristic of becoming orientable even when it is formed on the amorphous material. Therefore, platinum is used for forming the lower electrode because of the reason described in the above.
However, the conventional ferroelectric capacitor has following problem to be resolved.
The problems is degradation of ferroelectricity caused by frequent inversion of polarization, aging and leakage of oxygen from the ferroelectric substance (PZT), since platinum has a tendency of allowing oxygen and Pb to pass through it. In other words, there is high probability of leakage of oxygen and Pb contained in the ferroelectric substance through the columnar crystals of platinum shown in FIG. 11. The problem also arises in a capacitor using dielectric substance having high dielectric constant.